Historically, when microprocessors were first commercialized, each microprocessor held less than one million transistors, and operated at speeds below 20 MHZ. Typically, the microprocessors were either solder mounted onto a motherboard directly or mounted to the motherboard through sockets. They were typically air cooled, and air flow was typically sustained by a system fan. Because of the relatively slow operating speeds EMI and electrostatic discharge (ESD) shielding was not a high priority.
The number of transistors contained in each microprocessor, as well as the processor operating speed, have increased dramatically. Correspondingly, the amount of heat that needs to be dissipated, and the amount of EMI emissions that need to be absorbed have increased. Sensitivity to ESD during operations has also increased. As a result, most of today's microprocessor based computer systems employ either local fans and/or heat sinks to help ensure that the microprocessors will run cool. At the same time, increased attention has been given to the design of system chassis to ensure that adequate levels of EMI emission absorption and ESD resistance are met. The trend to increase transistor density and operating speed is expected to continue. It is expected that because of the amount of heat that needs to be dissipated, the precision of bonding between the processor package and a heat sink will reach a point of critical importance. That is, the physical bond between the processor package and the heat sink plays a critical role in heat dissipation.
EMI is produced in varying degrees by virtually every type of electrical equipment. Electromagnetic shielding is typically used to protect electrical equipment from unwanted electromagnetic radiation or EMI. Many systems such as processors require at least some shielding for proper operation or to meet EMI requirements for emissions and immunity. As circuit speeds and sensitivities increase so will the need for improved shielding. Shielding can be generally described as a conductive or ferromagnetic material which either reflects, absorbs or carries electromagnetic interference to ground.
Electromagnetic shielding often provides protection for electrical equipment by reducing unwanted signals to levels that do not adversely affect equipment. This is achieved by both reflecting and absorbing the radiation signals. Reflection depends on a permeability and conductivity of the shielding material, and a frequency and wave impedance of the signal. Generally, the reflectance of a shielding material increases with frequency. The remaining signal passing through a shielding material is reduced by absorption. The resistivity and thickness of the shield, effects absorption. A magnetic material is more often effective in absorption.
To deal with low-frequency EMI, thick steel shields have been used to absorb the energy. However, absorption is more effective if the shield is protecting the electrical equipment from frequencies that are high. In addition, a shield have been provided which use a "Faraday Cage" to protect the circuits inside the shield. A Faraday cage is commonly known as an earthed metallic wire or gauze screen enclosing electrical equipment to shield it from the influence of external electric fields.
Heat dissipation has been decreased by using clips which physically hold a heat sink to the processor package. One such clip is a Single Edge Processor Package (SEPP). The problem with SEPP's is that it does not suppress EMI.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved EMI shielding for computer processors.